docs/_includes/nav-left.html

{% assign navurl = page.url | remove: 'index.html' %}
<ul>
{% for item in include.nav %}
    <li>
        <a href="{{ rel_path_to_root }}{{ item.url }}">
            {% if navurl contains item.url %}
                <b>{{ item.text }}</b>
            {% else %}
                {{ item.text }}
            {% endif %}
        </a>
    </li>
    {% assign tag = item.url | remove: ".html" %}
    {% if item.subitems and navurl contains tag %}
        {% include nav-left.html nav=item.subitems %}
    {% endif %}
{% endfor %}
</ul>